Wednesday’s big snowstorm arrived ahead of schedule this morning, prompting New York City to declare an official weather emergency and sending the National Weather Service scrambling to up its previous snowfall forecast.

The Bronx’s Bedford Park neighborhood had already received a quick four inches by 11 a.m. Wednesday, and northeast New Jersey was not far behind at nearly three inches of fresh powder. There was no formal observation filed yet from Central Park, but any Manhattan office worker sitting near a window can attest to the steady snowfall since this morning. Flakes have been dropping in bunches as far south as Washington D.C. too, and thundersnow has been reported not far outside of town.

As Weather Journal noted Tuesday evening, there were reasons to expect this snowstorm might top consensus forecasts; we predicted up to a foot of snow by Thursday in the Bronx. But with quicker-than-expected accumulation coming so early in the storm’s lifecycle, even our prediction might prove too conservative.

What’s Going On? With the center of the “mother storm” still forming off the coast of North Carolina, the heaviest snow won’t impact our region until the overnight hours. The storm will then quickly grow to monster scale, drawing tropical moisture northwards, combining it with Canadian chill and converting it to snow at an impressive rate.

Expect accumulation rates to approach two inches per hour between 7 p.m. and 1 a.m., with lighter snow tapering off around daybreak on Thursday. The somewhat unexpected intensity of this storm could catch many people off guard.

Why did early forecasts underestimate this storm’s snow-generating potential? They seem to have trusted the computer models and figured that such a strong storm would surely bring more rain than snow. Here’s how the National Weather Service explained their hasty ratcheting up this morning:

WHOLESALE UPDATE TO NEAR TERM FORECAST TO ACCOUNT FOR MUCH QUICKER DEVELOPMENT OF HEAVY PRECIP. LIKELY CAUSE FOR POOR MODEL HANDLING WAS DUE TO PROLIFIC CONVECTION OVER THE GULF…INTERRUPTING MODEL HANDLING OF SUB TROPICAL MOISTURE FEED.

On the heels of the storm, it will feel absolutely freezing on Thursday and Friday. Cold, strong winds will sweep through the region, pushing temperatures into the teens.